Direct Detection Systems Research Articles

Bias-aided DD-LMS equalizer for optical multipath-interference-impaired high-speed PAM4 transmission systems

Multipath interference (MPI) noise induces drastic fluctuations in high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) systems, severely degrading the transmission performance. Here, we propose a bias-aided decision-directed least mean square (DD-LMS) equalizer to eliminate the MPI noise. In the simulation, the proposed bias-aided DD-LMS equalizer could adeptly track and compensate the MPI-impaired PAM4 signal, markedly improving the bit error rate (BER) performance under different MPI levels and laser linewidths. Within a 112 Gbps PAM4 transmission system, the proposed equalizer achieves a MPI tolerance over 4 dB at the KP4-forward error correction (FEC) threshold (2.4 × 10−4), outperforming existing MPI suppression techniques.

Demonstration of a low-complexity real-time FBMC transmitter for a spectral-efficiency IMDD system.

Filter bank multi-carriers (FBMC) have higher spectral efficiency, lower out-of-band spectrum leakage, and stronger ability to resist synchronization errors compared to orthogonal frequency-division multiplexing (OFDM), which has been widely considered as a promising solution for short-reach applications, such as passive optical networks. However, the traditional fast Fourier transform (FFT)-based FBMC scheme has quite high implementation complexity for high-speed optical communications. In this work, we proposed a low-complexity real-time FBMC transmitter scheme enabled by a single fully parallel pruned inverse FFT and simplified short prototype filter-based polyphase networks for a low-cost intensity-modulation and direct-detection (IMDD) system. Compared to the OFDM transmitter, the proposed FBMC one occupies a similar number of look-up tables and registers but saves 21% of real multipliers, reduces latency by 16.2%, and increases the data rate by 12.5%. The real-time FBMC/OFDM signals are generated with a field programmable gate array chip and 6-bit 30-GSa/s digital-to-analog converter and are experimentally demonstrated in an IMDD system. After a 20-km standard single-mode fiber transmission, the negligible power penalty can be achieved with the proposed FBMC transmitter scheme at the bit error rate of 3.8e-3 compared to the OFDM counterpart.

Enhanced ABLO-OFDM performance by using composite multi-mode index modulation in IM/DD systems

In intensity modulation direct detection (IM/DD) systems, an adaptively biased layered optical orthogonal frequency division multiplexing (ABLO-OFDM) scheme is proposed based on composite multi-mode index modulation (CMM-IM). The ABLO-OFDM has a simple structure, easy demodulation, low complexity, and effectively reduces the peak-to-average power ratio (PAPR). Since CMM-IM extends the index to the constellation domain, it increases the number of index bits to further improve the spectral efficiency (SE). By dynamically adjusting the number of activated subcarriers and the number of layers, various signal power can be achieved to enhance system flexibility and reliability. The system performance of four schemes is compared and analyzed in IM/DD systems. The result shows that, for the same SE, CMM-ABLO (4,3) achieves a 3 dB receiver sensitivity gain compared with CMM-ABLO (8,2). When L is 3, four schemes can transmit over about 50 km single-mode fiber under the soft decision forward error correction (SD-FEC) threshold. When L is 1 and the sample rate is less than 20 GSa/s, both CMM-ABLO (4,2) and CMM-ABLO (4,3) can achieve error-free transmission. Moreover, as the number of layers increases, PAPR decreases and it reaches a maximum reduction of 9.6 dB.

FPGA implementation of power-lite Volterra-inspired neural network equalizer in 180-Gb/s net bitrate IMDD short-reach optical system.

A white-box power-lite Volterra-inspired neural network (VINN) equalizer is proposed to solve the problem of complexity discontinuity in a Volterra nonlinear equalizer (VNLE). By adjusting the granularity of the solution space, it conserves computational resources while maintaining nonlinear compensation capability. The performance of VINN is verified on a field-programmable gate array (FPGA) in a short-reach intensity modulation and direct detection (IMDD) system, and a 240-Gb/s real-time signal processing rate is achieved. Under the 25% overhead soft-decision forward error correction (SD-FEC) bit error rate (BER) threshold, we realize a record net rate of up to 180 Gb/s based on the FPGA.

Concurrent performance and security enhancement of ultrahigh-order DSM signals via null subcarriers.

In this paper, we propose leveraging null subcarriers in discrete multi-tone modulation (DMT) to process the DMT signal in both time and frequency domains. Additionally, we employ discrete memory enhanced chaos (DMEC) to scramble the signal in the frequency domain, thereby achieving physical layer signal encryption while ensuring a more uniform power distribution in the time-domain waveform. In our experimental demonstration, we achieved high-security transmission of a DSM-based 65536-QAM signal at a data rate of 16.01 Gb/s over a 25 km single-mode fiber (SMF) in an intensity-modulation direct-detection (IMDD) system. Additionally, in the transmission experiments for 13684-QAM and 65536-QAM signals, the proposed method demonstrated a receiver sensitivity gain of over 0.5 dB compared to the traditional DSM-based ultrahigh-order transmission.

Neural network-based electrical dispersion pre-compensation for a 56 Gb/s PAM-4 over an 80 km fiber in intensity-modulation and direct-detection systems.

A neural network (NN)-based electrical dispersion pre-compensation (pre-EDC) scheme in intensity-modulation and direct-detection (IM/DD) systems is proposed and experimentally demonstrated in this Letter. The scheme enables 56 Gbit/s four-level pulse amplitude modulation (PAM-4) generation at a transmitter over an 80 km single-mode fiber (SMF) transmission in the C-band. The NN is utilized to better fit nonlinear phase-amplitude transformation due to the chromatic dispersion (CD) in IM/DD systems, in place of the existing Gerchberg-Saxton (GS) iterative algorithm and linear GS-based finite impulse response (GS-FIR) non-iterative compensation schemes. The experimental results show that the measured bit error ratio (BER) can be reduced to below the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 with 0 dBm receiver optical power (ROP) by the NN-based non-iterative pre-EDC scheme, which also saves up to 81% of computational complexity compared to the GS-based scheme. The results indicate that our proposed scheme is promising for the CD pre-compensation at the transmitter.

Performance Investigation of Joint LUT and GS Algorithm at the Transceiver for Nonlinear and CD Compensation

In order to meet the increasing requirements of speed and distance, an advanced digital signal processing (DSP) algorithm is preferred without changing the system structure in intensity modulation and the direct detection (IM/DD) system. As the transmission distance increases, the power fading induced by dispersion must be mitigated. In addition, linear and nonlinear inter symbol interference (ISI) introduced by bandwidth limitation and device imperfections becomes an obstacle to achieving higher capacity. The Gerchberg–Saxton (GS) algorithm was recently used to compensate for dispersion. In this paper, GS-based pre- and post-compensation schemes in the IM/DD system with nonlinearity were investigated. We investigated and compared the performance of the GS-based pre- and post-compensation algorithm in a 28 GB aud four-level pulse amplitude modulation (PAM-4) transmission over 40 km standard single-mode fiber (SSMF). The bit error rate (BER) achieved a threshold of 3.8 × 10−3 using look-up-table (LUT), FFE, and the GS-based pre-compensation algorithm without iterations. Turning to the GS-based post-compensation scheme, 80 iterations are needed. However, the demand for FFE is reduced. The algorithm selection depends on the tolerance of the transmitter or receiver complexity in specific scenarios. The joint LUT and GS-based pre-compensation algorithm may be a preferable approach in scenarios where a low-complexity receiver is desired.

Direct detection system for full-field nanoscale X-ray diffraction-contrast imaging

Recent developments in X-ray science provide methods to probe deeply embedded mesoscale grain structures and spatially resolve them using dark field X-ray microscopy (DFXM). Extending this technique to investigate weak diffraction signals such as magnetic systems, quantum materials and thin films prove challenging due to available detection methods and incident X-ray flux at the sample. We present a direct detection method developed in conjunction with KAImaging which focuses on DFXM studies in the hard X-ray range of 10s of keV and above capable of approaching nanoscale resolution. Additionally, we compare this direct detection scheme with routinely used scintillator-based optical detection and achieve an order of magnitude improvement in exposure times allowing for imaging of weakly diffracting ordered systems.

Complex-valued recurrent neural network equalizer with low complexity for a 120-Gbps 50-km optical PAM-4 IM/DD system

This paper introduces a novel complex-valued recurrent neural networks equalizer (RNNE) designed for a 120-Gbps, 50-km optical 4-level pulse-amplitude modulation (PAM-4) intensity modulation and direct detection (IM/DD) system. By mapping adjacent symbols of PAM-4 signals onto the complex domain, the correlation between two adjacent symbols of PAM-4 signals can be preserved. Based on experimental results, the proposed complex-valued RNNE outperforms the traditional real-valued RNNE with a 1.38-dB system power budget gain at the 7% overhead forward error correction BER threshold of 3.8 × 10−3. We believe that complex-valued RNNE has an advantage over real-valued RNNE in processing real-valued signals in IM/DD systems.

Sub-rate sampled, non-integer fractionally spaced Volterra nonlinear equalizer for IM/DD systems

As high-speed analog-to-digital converters (ADC) account for a significant portion of the receiver’s cost in intensity-modulation (IM)/direct-detection (DD) systems, there have been substantial efforts to employ an ADC operating at a relatively low sampling rate. However, half-symbol-spaced electronic nonlinear equalizers used to compensate for nonlinear waveform distortions commonly operate at 2 samples/symbols, and thus require digital upsampling before the equalization. This implies that the digital signal processing (DSP) at the receiver should also operate at 2 sample/symbol even though the ADC operates at the sub-rate (i.e., < 2 sample/symbol). Hence, we propose and experimentally demonstrate the sub-rate sampled, non-integer fractionally spaced Volterra nonlinear equalizer (VNLE). This equalizer does not require the digital upsampling, and thus makes the entire receiver DSP block operating at the sub-rate, the same as the sampling rate of ADC. We estimate the complexity of the proposed equalizer by the number of multipliers required for its implementation. We also evaluate the performance of the proposed VNLE over a 64-Gb/s 4-ary pulse amplitude modulation link and compare the performance with the conventional VNLE (requiring digital upsampling). The results show that the proposed VNLE incurs a very slight performance degradation compared with the conventional VNLE, but reduces the implementation complexity considerably since it obviates the need for digital upsampling at the receiver.

Low complexity sub-baud rate sampling reception in a bandwidth-limited IM/DD system.

This Letter presents what is to our knowledge a novel approach to reduce the digital signal processing (DSP) complexity in intensity modulation and direct detection (IM/DD) systems, which is critical for short-reach optical communication systems with severe bandwidth limitations. We propose a sub-baud rate sampling reception method utilizing a polyphase feedforward equalizer-based maximum likelihood sequence estimation (PFFE-MLSE), which could operate effectively under a sampling rate of 0.6 samples per symbol. This new architecture eliminates the need for resampling, allowing the adaptive equalizer to operate with significantly reduced complexity-over 60% compared to traditional FFE-MLSE. An offline experiment, transmitting a 100-Gbaud on-off keying (OOK) signal over a 5-km single-mode fiber (SMF) link, demonstrates the feasibility of our approach with bit error ratio (BER) meeting the KP4-forward error correction (KP4-FEC) threshold in the optical back-to-back (OBTB) scenario and 7% hard-decision FEC (HD-FEC) threshold in the 5-km SMF transmission.

Optimized equalization based on MAP detection for C-band bandwidth-limited IM/DD systems.

Entangled dynamic and deterministic inter-symbol interferences (ISIs) induced by complicated channel impairments, limit the transmission capacity of intensity modulation and direct detection (IM/DD) systems. This Letter proposes a colored noise-suppressed channel shortening filter (CNS-CSF)-enabled maximum a posteriori (MAP) estimation (CNS-CSF-MAP) scheme to disentangle and mitigate deterministic and dynamic ISIs, where the CNS-CSF is deployed to perform the optimized dynamic ISI equalization with equalization-enhanced noise suppression, and the subsequent MAP algorithm is used to eliminate the residual deterministic ISI. The performance of the CNS-CSF-MAP scheme is evaluated and demonstrated in a C-band 61-Gb/s 100-km optical on-off keying (OOK) IM/DD system. The experimental results show that the proposed CNS-CSF-MAP scheme reaches the 20% and 7% forward error correction (FEC) thresholds at received optical powers (ROPs) of -6.6 dBm and -4 dBm, achieving 0.5- and 1.5-dB gains over a conventional post-filter-enabled MAP (PF-MAP) scheme.

Weight-adaptive joint mixed-precision quantization and pruning for neural network-based equalization in short-reach direct detection links.

Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations. The NN connections are either directly cutoff or represented by a proper number of quantization bits by weight partitioning, leading to a hybrid compressed sparse network that computes much faster and consumes less hardware resources. The proposed approach is verified in a 50-Gb/s 25-km pulse amplitude modulation (PAM)-4 IM/DD link using a directly modulated laser (DML) in the C-band. Compared with the traditional fully connected NN-based equalizer operated with standard floating-point arithmetic, about 80% memory can be saved at a minimum network size without degrading the system performance. Quantization is also shown to be more suitable to over-parameterized NN-based equalizers compared with NNs selected at a minimum size.

Enhancing secure transmission of ultra-high order QAM via delta-sigma modulation integrated with radial basis function neural network enhanced chaos

This paper proposes and experimentally demonstrates an ultra-high order signal security enhancement method based on delta-sigma modulation (DSM). We employ a radial basis function neural network (RBFnn) to enhance Henon chaos, effectively avoiding the degradation of chaotic dynamics, and increasing the key space from 1054 to 10385. In the experimental demonstration, we achieved high-security transmission of 16384QAM over 25 km in an intensity-modulated direct detection (IMDD) system based on DSM. The experimental results show that the ultra-high order transmission scheme based on RBFNN-chaos and DSM has superior security, with a key sensitivity of up to 10−16.

Enhancing secure transmission and key distribution for ultrahigh-order QAM via delta-sigma modulation and discrete memristive-enhanced chaos.

In this Letter, we propose a method for ultrahigh-order QAM secure transmission and key distribution based on delta-sigma modulation (DSM) and discrete memristive-enhanced chaos (DMEC). The disturbance vectors generated by the DMEC scramble the DSM signals in both frequency and time domains, resulting in highly secure DSM signals. Through the key modulation and power adjustment and then superimposing them on the encrypted signals, the method achieves simultaneous transmission of keys and signals without the need for additional spectral resources. This approach allows for secure communication with continuous key iteration and updates, offering an effective solution for implementing "one-time pad" encryption. In the experimental demonstration, we achieved a secure transmission and key distribution of a 16384QAM signal at a rate of 17.09 Gb/s over 25 km in an intensity-modulated direct detection (IMDD) system, based on DSM.

A cascaded bit-weighted distribution matching for LDPC-coded PS-64QAM DMT in a high-speed Unamplified IM-DD system

We experimentally demonstrate the generation and transmission of probabilistic shaping (PS) 64-ary quadrature amplitude modulation (64-QAM) discrete multi-tone (DMT) signal with low-density parity-check (LDPC)-coded modulation based on two-stage bit-weighted distribution matching (TS-BWDM) in a high-speed unamplified intensity modulation and direct detection system. The TS-BWDM scheme is based on a two-stage bit-weighted inversion algorithm, which has the advantages of no complex multiplication and division operations and low hardware implementation complexity. As the key content of TS-BWDM, before FEC coding, the bit-weighted intervention operation is used to change the bit probability of a specific sequence in the parallel binary sequence. The two-stage bit-weighted inversion operation and one-stage weight-labeling bit insertion is performed for achieving the target symbol probability distribution. As a result, it does not add a significant amount of extra redundant bits. Additionally, the high-accuracy training symbols-based sampling frequency offset (SFO) estimation method and digital interpolation are used to compensate SFO. In our experiments, the TS-BWDM-based PS-64QAM DMT signals with a net rate of 157.3-Gb/s transmission over 10-km non-zero dispersion- shifted fiber can be achieved. The system performance is investigated under two LDPC code rates (3/4 and 9/10) and three PS parameter values (k = 3, 5 and 9). The experimental results show that the receiver power sensitivity and the system fiber nonlinear effect tolerance can be significantly improved compared with uniformly-distributed signals.

Low-complexity frequency domain frame synchronization for short-reach IM/DD optical fiber transmission systems

We propose a low-complexity frequency domain frame synchronization method for short-reach intensity modulation and direct detection (IM/DD) systems. A four-level pulse amplitude modulation-training sequence (PAM4-TS) is specially designed for the proposed method, which has an obvious peak in the amplitude spectrum that is higher than the normal signal. The proposed method comprises a coarse synchronization stage and a fine synchronization stage. Firstly, the coarse synchronization stage takes advantage of the feature of PAM4-TS to obtain the approximate position of the frame head by identifying the peak value in amplitude spectrum of the segmented received signal. Then, the fine synchronization stage calculates the correlation between the coarse synchronization result and PAM4-TS by multiplying the two in the frequency domain. Compared with the traditional sliding window correlation method realized in the time domain, both simulation and experimental results of C-band 50 Gbit/s PAM4 transmission demonstrate that the proposed method reduces the multiplication complexity by up to about 96.01% without any additional performance penalty.

Planning tools for next-generation DSP-based passive optical networks above 50G [Invited Tutorial

Next-generation optical access networks are evolving towards ultra-high bit rates (above 50 Gbps per wavelength) and extended fiber reach architectures. This trend will likely push the optoelectronics to their limits, thus requiring impairment compensation based on digital signal processing (DSP) techniques in the transceivers. In this paper, which is an invited follow-up of a tutorial given at ECOC 2023, we first present an overview of this evolving scenario and then propose a unified analytical model that is able to predict the performance of these new systems for both direct-detection and coherent transceiver types. We believe that this model can be useful for preliminary scalability studies of new access architectures (as it happens in international standardization bodies). Moreover, when they are deployed, it can be useful as a base for network planning tools, particularly if future transceivers will be, as expected, highly reconfigurable at the DSP level.

Extending the coverage of user-diversified IM-DD PON by FDM: a mathematical model with experimental verification.

Future passive optical networks (PONs) call for more flexibility to support diversified users with various rate demands and link qualities. Using traditional time-division multiplexing (TDM), the concept of a flexible rate PON was proposed to accommodate more users with link diversity by rate adaptation. In this Letter, we reveal the PON coverage can be further extended through frequency-division multiplexing (FDM) in the presence of multiuser diversity, namely, (i) there exist users with frequency-dependent link conditions and (ii) the link conditions exhibit disparity among users. We build a mathematical model and propose an optimization algorithm based on the binary tree search to optimize diversity gain. We experimentally verify its feasibility by studying the diversity gain concerning chromatic dispersion, optical path loss, and signal-to-noise ratio (SNR) variation in a 200G-class intensity-modulation direct-detection (IM-DD) system.

Dual high-order QAM-modulated mm-wave signal transmission in the Q-band enabled by simple IM/DD architecture and bandpass delta-sigma modulation.

The intensity-modulation (IM)/direct-detection (DD) systems have been proven effective and low-cost due to their simple system architecture. However, the Mach-Zehnder modulator (MZM) of the IM/DD systems only reserves its driving signal intensity. Therefore, the IM/DD systems are generally unable to transmit vector signals and have a restricted spectrum efficiency and channel capacity. Similarly, the radio-over-fiber (RoF) transmission systems based on IM/DD are limited by their simple architecture and generally cannot transmit high-order quadrature amplitude modulation (QAM) signals, which hinders the improvement of their spectrum efficiency. To address the challenges, we propose a novel, to the best of our knowledge, scheme to simultaneously transmit the dual independent high-order QAM-modulated millimeter-wave (mm-wave) signals in the RoF system with a simple IM/DD architecture, enabled by precoding-based optical carrier suppression (OCS) modulation and bandpass delta-sigma modulation (BP-DSM). The dual independent signals can carry different information, which increases channel capacity and improves spectrum efficiency and system flexibility. Based on our proposed scheme, we experimentally demonstrate the dual 512-QAM mm-wave signal transmission in the Q-band (33-50 GHz) under three different scenarios: 1) dual single-carrier (SC) signal transmission, 2) dual orthogonal-frequency-division-multiplexing (OFDM) signal transmission, and 3) hybrid SC and OFDM signal transmission. We achieve high-fidelity transmission of dual 512-QAM vector signals over a 5 km single-mode fiber (SMF) and a 1-m single-input single-output (SISO) wireless link operating in the Q-band, with the bit error rates (BERs) of all three scenarios below the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. To the best of our knowledge, this is the first time dual high-order QAM-modulated mm-wave signal transmission has been achieved in a RoF system with a simple IM/DD architecture.